Field of the Invention
The present invention relates to a stock solution composition for fuel cell vehicle coolant used by dilution with a diluent comprising water and a method for producing the same, and further to a fuel cell vehicle coolant composition having improved storage stability and a method for producing the same.
Background Art
Generally, a stack in a fuel cell vehicle is a stacked structure of a plurality of single cells, and a cooling plate for cooling the stack (single cells) is interposed for each substack composed of several single cells. A coolant path is formed inside the cooling plate, and the stack is cooled by the flow of a coolant through the coolant path. In this manner, the coolant in the fuel cell vehicle is circulated in the stack generating power, that is, between the substacks, and therefore, in order to prevent electrical leakage outside the stack and a decrease in power generation efficiency (reduction in energy loss) caused by the resistance of the coolant, high insulation performance is required. In order to satisfy requirements such as the ensuring of the insulation performance and the ensuring of cooling efficiency, pure water has been used as the coolant in conventional art. But, considering, for example, automotive fuel cells or fuel cells for home cogeneration system, the temperature of the coolant decreases to ambient temperature during non-operation. Particularly, when there is a possibility of use below the freezing point, pure water may freeze to impair the cell performance of the fuel cells, such as the breakage of the cooling plates due to coolant volume expansion.
Because of such circumstances, the use of glycol compounds for the purpose of nonfreezing properties is considered, but even in the case where these are used as the base of a fuel cell vehicle coolant, a problem is that low conductivity cannot be maintained because the conductivity increases. The increase in conductivity during storage before filling to vehicle is a problem for the fuel cell vehicle coolant of which high insulation performance is required in order to prevent electrical leakage to the outside. It has been considered that the increase in the conductivity of the coolant is caused by the oxidative degradation of the glycol compound due to the heat load of the running of the vehicle, and therefore, measures have been taken by the blending of an antioxidant.
For example, JP Patent Publication (Kokai) No. 2008-059990 A, JP Patent Publication (Kokai) No. 2008-059988 A, JP Patent Publication (Kokai) No. 2012-009263 A, and JP Patent Publication (Kokai) No. 2008-059825 A disclose that the oxidative degradation of an aqueous solution of an ethylene glycol compound is inhibited by adding a cycloalkylamine or a derivative thereof (for example, cyclohexylamine) (JP Patent Publication (Kokai) No. 2008-059990 A), thiourea or a derivative thereof (for example, thiourea dioxide) (JP Patent Publication (Kokai) No. 2008-059988 A), a particular aromatic group-containing glycoside compound (for example, salicin) (JP Patent Publication (Kokai) No. 2012-009263 A), or a polyamine (for example, hydrazine) (JP Patent Publication (Kokai) No. 2008-059825 A). However, the effect of inhibiting conductivity increase during long time storage by blending such an antioxidant is not sufficient.
In addition, JP Patent Publication (Kokai) No. 2009-073999 A discloses the use of a nonaqueous solvent having low conductivity such as a fluorine-based solvent. However, the nonaqueous solvent has the following problem: the non nonaqueous solvent has a poor heat exchange efficiency because of its low specific heat, and therefore, an increase in radiator size is required, which is not practical.
As described above, it has been required to find a technique for inhibiting conductivity increase during long time storage in a fuel cell vehicle coolant containing water and a glycol advantageous in terms of cooling efficiency and nonfreezing properties, but this has been in a difficult situation.